1. Field of the Invention
The present invention relates to an f.theta. lens system in an optical scanner using a rotary polygon mirror.
2. Description of the Related Art
An optical scanner is known as a device for writing and reading information by performing an optical scanning operation of a light beam and is used in a laser printer, a facsimile, etc.
In such an optical scanner, there is a device of a system in which an approximately parallel light beam from a light source device is formed as a line image extending in a main scan-corresponding direction and is deflected at an equal angular velocity by a rotary polygon mirror having a reflecting face in the vicinity of the position of the formed line image. In this system, the deflected light beam is formed as an image in the shape of a spot on a scanned face by an image forming lens system to scan the scanned face.
In the optical scanner using the rotary polygon mirror, there is a problem about a so-called reflecting face inclination. Further, since the angular velocity of the rotary polygon mirror is constant, the scanned face is not scanned by the deflected light beam at a constant speed when a normal f.tan .theta. lens is used. Accordingly, it is necessary to dispose a means for performing the optical scanning operation at an equal speed. The f.theta. lens system is a lens system for performing the optical scanning operation at a constant speed with respect to this scanned face. This lens system has an f.theta. function constructed such that the height of an image formed by the light beam having an incident angle .theta. with respect to a lens optical axis is set to f.theta. when the focal length of the lens system is f.
As a method for solving the problem about the reflecting face inclination, there is a method in which a lens system disposed between the rotary polygon mirror and the scanned face is set to an anamorphic system and the scanned face and a light reflecting position of the rotary polygon mirror are set in a conjugate relation in geometrical optics with respect to a cross scanning direction.
There are various kinds of lens systems in which the f.theta. lens system is set to an anamorphic system to solve the problems about the optical scanning operation at a constant speed and the reflecting face inclination. For example, Japanese Patent Application Laying Open (KOKAI) No. 63-19617 shows an f.theta. lens system having two lenses. However, in this f.theta. lens system, field curvature is not necessarily corrected sufficiently and the diameter of a formed image spot on the scanned face is considerably changed in accordance with a scanning position so that it is difficult to perform the optical scanning operation of high density. Japanese Patent Application Laying Open (KOKAI) No. 61-120112 shows an f.theta. lens system having two lenses and using a so-called saddle type toric face to preferably correct field curvature. However, in this f.theta. lens system, two aspherical lens faces are used so that it is difficult to process the lenses and manufacturing cost is high.